This invention relates to the formation of expanded and specially contoured socket-end portions, frequently referred to as "bells", on lengths of pipe or tube composed of relatively rigid, thermoplastic material, and has particular reference to the thickening of the belled end portions of such pipes.
Plastic pipe, typically composed of polyvinylchloride (PVC), is becoming increasingly popular for a wide variety of uses, and the production of such pipe has developed into a substantial industry. At present, PVC pipe is extruded in straight, cylindrical form and in a variety of preselected diameters and wall thicknesses, and is cut to sections of selected length. One end of each section is belled, that is, expanded and formed to a special contour, to receive the unexpanded end portion of another section of pipe in telescoped and sealed relation, as part of a pipe line.
To seal the telescoped joints in a line of pipe, sealing gaskets are fitted between the telescoped portions of the pipe sections in internal annular grooves that are formed in raised external ribs on the belled end portions, to encircle and seal against the internal cylindrical end portions. Such grooves may be designed with various cross-sectional configurations, to receive gaskets of various designs, and typically are formed by deforming an annular portion of the plastic adjacent the end of the pipe section radially outwardly during the belling operation and then holding the plastic in the desired shape as it is cooled and sets.
There are several available machines for belling plastic pipe sections, one well-known type being shown in Kuhlemann U.S. Pat. No. 3,553,780, and another being shown in Niessner et al. U.S. Pat. No. 3,264,383. Another example of such a belling machine is shown in Wilson et al. application Ser. No. 512,230, filed Oct. 4, 1974.
Each of these belling machines utilizes a shaped mandrel which is inserted in the end portion of a plastic pipe section while the end portion is in a heated, thermoplastic state, to expand the end portion to the desired belled shape, either as an incident to the movement of the pipe section onto the mandrel or by expansion of a collapsed mandrel after it is within the pipe section. An external split mold ring frequently is provided to be clamped around the expanded pipe section, at least in the area of the seal rib, to clamp the pipe against the mandrel and determine the outside configuration of the bell.
Typically, the resulting bell has a short cylindrical lip or land adjacent its belled end, a raised annular rib, a corresponding internal gasket groove inside the rib, and a relatively long barrel portion beyond the groove with an inside diameter sized to receive the cylindrical end portion of the adjacent pipe section. This barrel portion is integrally joined to the cylindrical body of the pipe section by a short taper.
Standard specifications for such pipe sections prescribe wall thicknesses that should be provided for different pressure ratings of the pipe and for different pipe sizes, and sometimes specify that the wall thickness should increase with increases in the diameter of the pipe. Thus, as the diameter of the belled end portion of a pipe section increases, the wall thickness should increase correspondingly.
With conventional belling machines, however, the opposite condition is produced. Because the plastic is stretched radially outwardly as the pipe section is expanded, the largest-diameter portion of each section (the annular rib defining the gasket groove) has the thinnest wall, substantially thinner than the initial wall thickness, rather than the thickest wall, as prescribed by the standard specifications.
Concern regarding the reduced strength of the belled end portions of the pipe sections has resulted in efforts to reinforce the bells, either through thickening of the plastic walls or through the addition of extra layers of reinforcing material. The aforementioned Niessner patent discloses a method for extruding pipe with axially spaced, thickened sections which later will be expanded to form bells. Kuhlemann U.S. Pat. No. 3,557,278 discloses a method of thickening a bell by axial compression of the bell during the belling operation. An example of a laminated bell is shown in Parmann U.S. Pat. No. 3,887,992.
Thus, methods of reinforcing the bells of thermoplastic pipes are known, perhaps the most widely accepted being those disclosed in U.S. Pat. No. 3,264,383 and U.S. Pat. No. 3,557,278. Unfortunately, these methods either require relatively complex machinery or are relatively expensive, particularly in view of the consideration required by the patentees for use of the respective inventions. In addition, these known methods frequently are difficult to control, from the viewpoint of radial dimensional tolerances or axial location of raw material, because excessive thicknesses, or excessive axial length of thickened portions, are provided to insure that sufficient thickness will be provided in the critical areas.
Accordingly, a principal objective of the present invention is to provide a new and different method of thickening the belled-end portions of pipe sections, which method is beyond the scope of the aforesaid patents and which, at the same time, is relatively simple and inexpensive to use, and provides close control of the location of the thickened portions.